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Neutral Meson Production at High p T with the PHENIX Experiment at RHIC

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Heavy Ion Collisions at the LHC

ECT*, Trento

September 1, 2006

Neutral Meson Production at High pT

with the PHENIX Experiment at RHIC

Henner Büsching

FIAS / University of Frankfurt

Council of Trent

1545-1563

=

18 Years !

response to

the theological

challenges of the

Protestant Reformation

Let’s hope we come to conclusions a lot faster!

!

The p0 Analysis in PHENIX

- The observables:
- Important part of one of the first PHENIX papers (paper #3)
- Highest citations of all PHENIX papers:
- Phys.Rev.Lett.88:022301,2002 : Spires 335 citations
- Phys.Rev.Lett.91:072301,2003 : Spires 258 citations
- PHENIX is analysing p0 for 6 years now
- p0 analysis one of the PHENIX working horses
- It still needs some time to publish

Fast first results

High impact

Long experience

Established analysis scheme

New challenges with every run

- Running long enough for critical review
- What can we learn ?

Fast first results

High impact

?

Why?

How?

What?

Long experience

Established analysis scheme

New challenges with every run

A Good General Case Study:

The p0 Analysis in PHENIX

High impact

The p0 Analysis in PHENIX

- In principle a simple analysis
- Self calibrating
- Self identifying
- Experience from previous experiments
- PbGl detector re-used from WA98
- Two independent detectors
- Two analysis groups – cross check

- Strong discovery
- Predictions available at start
- Theory-friendly (easy to calculate)
- Identified particle
- High pT

Long experience

- Calorimeter “veterans” from previous experiments
- New expertise developed in PHENIX
- Human resource development
- Young people start
- Experienced people can move on
- No gaps in analysis strategy
- High interest from non pure p0 analyses
- Direct photon
- Spin
- Correlations
- Guarantees fresh ideas and critical perspective
- Different reaction systems / energies
- New data sets – changing detectors

Established

analysis scheme

New challenges

with every run

First years : main discoveries

1

2

3

g

p0

Phys. Rev. Lett. 91, 072301 (2003)

Phys. Rev. Lett. 91, 072303 (2003)

Phys. Rev. Lett. 94, 232301 (2005)

Are we done? – No

- Improve p+p reference
- New Data:
- Better Centrality dependence
- Higher pT reach
- System size dependence
- Energy dependence
- Particle species dependence

- Better Understanding of :
- Influence of initial state effects
- Influence of final state effects

Signal

- h can be measured at high pT in PHENIX
- pT (h, p0) ≥ 1 GeV/c
- bulk: GeV/c
- Should hadronize in vacuum
- Neutral meson with 4 x mass of p0
- Second largest source for
- decay photons
- decay e ±
- Important for
- direct photon
- single electron + dielectron

Background

p0 spectra

Cu+Cu 200 GeV

56 M min-bias events

1.9 M high-pT events

2.2 B sampled

- Au+Au 200 GeV
- Luminosity 241b-1 (sampled)
- 1.5B events

Run5 Data

Run2 Data

(New) PHENIX

paper

!

nucl-ex/0601037

- Au+Au 200 GeV
- 34 M minimum-bias events
- + 30 M high-pT
- (LVL2 events) sampled

RAA in AuAu at 200 GeV

g

Run4 0 Data

p 0

h

Photons are not suppressed

p0 and h even at high pT suppressed

Suppression is flat at high pT

!

Au+Au

d+Au

p+p

Similar suppression pattern p0 and h

h/p0 ~ 0.4 - 0.5. in all systems and for all centralities

Universal suppression for light mesons

Suppression at partonic level

before fragmentation !(?)

!

p 0

Au+Au Cu+Cu

Au+Au 30-40 %, NPart = 114.2

Cu+Cu 0-10 %, NPart = 98.2

Similar suppression at similar NPart

Systematic comparison possible

!

p 0

- Close to SPS Energies
- p+p data at 21.7 – 23 GeV
- Use of parameterization as reference
- 3 days of RHIC Run5
- 6.8M Events after quality cuts
- Centrality via PC1 multiplicity

RAA – Energy dependence

Vitev

nucl-th/0404052

Au+Au

pp ref:

D. d’Enterria

Cu+Cu

dNg/dy=650-800

Vitev

nucl-th/0404052

62 GeV

22.4 GeV

Now we can study influence of collision energy

on scaling behavior

!

p0

- Mass
- 135.0 MeV
- Decay Modes
- 2g 98.8%
- e+e- g 1.2%
- Mean life
- 8.4*10 -17 sec
- cτ = 25 nm
- 40 times nuclear radius
- leaves collision zone before it decays
- 1/250,000 BBC resolution
- decays at measured vertex position

- p0,h via p0,h gg:
- Lead-scintillator calorimeter
- Lead-glass calorimeter
- Centrality, vertex
- Beam-Beam Counter (BBC) 3.0 < |h| < 3.9
- Zero-Degree Calorimeter

g

p0,h

g

(pseudorapidity |h| < 0.35)

PbSc tower

66 sampling cells

1.5 mm Pb, 4 mm Sc

penetrating wavelength shifting fibers

for light collection

PbSc towers:

5.52 x 5.52 x 33 cm3

(18 X0)

6 sectors with

15552 blocks total

PbSc

- Electrons and Photons:
- Bremsstrahlung, pair production
- Electromagnetic shower
- Strongly interacting particles:
- Hadronic shower, MIP
- Calorimeter measures energy,
- position, and TOF

Fiber

Scintillator

Pb +

generate

shower

generate

light

collect

light

- Charged shower particles generate Cherenkov photons in the PbGl
- The Ch. Photons propagate with a wavelength dependent attenuation to the PMT

homogeneous lead-glass

Cherenkov radiator

PMT

PbGl

- p0 -> 2g
- E2 = p2 + minv2 (c=1)
- Conservation of energy and momentum
- Valid for both p0 and 2g system
- E2(2g) – p2(2g) = minv2 (p0)
- E(2g) = E(g1) + E(g2)
- (2g) = (g1) + (g2)
- Take any two photons in event
- Calculate minv
- If minv = 135 MeV -> p0

- Alternative formula: minv2 = 2E1E2(1-cosψ)
- Not good to calculate minv
- cos needs more CPU than vector addition!
- But illustrates
- the higher the p0 pT the smaller the opening angle
- natural limit of p0 measurement

- High pT
- From 10 (15) GeV on
- clusters start to merge
- Beyond 25 (30) GeV
- photons overlap completely
- look like single photon

- Low pT:
- nonlinearity of EMCal response
- corresponding uncertainty
- p0 spectra so far only starting from 1 GeV
- Going to lower pT might be possible but is challenging

- There are less eta mesons compared to p0
- e/p ratio ~ 0.5
- The branching ratio into photons is smaller (40%)
- Mass is higher compared to pions:
- For given pT, opening angle is bigger
- At low pT - harder to hit the detectors
- At high pT – easier to measure as merging starts later

- Photon PID Cuts
- Asymmetry cut on pairs of photons
- Invariant Mass Distribution
- Mixed Event Background Subtraction
- Acceptance + Efficiency Corrections

PID Cuts on Photons

- p0 peak is the best Particle Identification criterion one can ask for
- Photon PID not mandatory
- So why bother with additional PID cuts?
- hadrons contribute to background
- Getting rid of hadrons
- increases signal/background ratio
- decreases statistical error of p0 yield

- Comparison of different PID’s to estimate systematic uncertainty

PID: Energy Cut

- At low energy calorimeter response is nonlinear
- Shower maximum close to detector surface
- in case of PbGl
- Cherenkov photons have to travel all the way through lead glass
- absorption
- Nonlinearity not known well enough (simulations)
- large uncertainty on energy

We can use PID cuts to eliminate

detector disadvantages – and optimize

!

PID: Shower Shape

- Hadron shower: λint > X0
- Spread of hadronic shower larger
- longitudinally
- laterally
- Lateral shower spread used to reject hadronic showers

Example for PID cuts to use

detector design

!

- Asymmetry cut α
- Energy asymmetry of photon pair
- pairs from p0 decays
- Flat asymmetry distribution
- random orientation of decay axis relative to p0 momentum

- Random pairs
- asymmetric energies favored
- Reason
- steeply falling photon energy spectrum
- many low-energy photons available to form random pairs

Asymmetry cut increases signal/background

!

p0 Reconstruction

- Identified by minv of decay-photon pair
- Which two photons in event originate from p0 ?
- All possible combinations of photon pairs
- Background of pairs that randomly have right invariant mass

Mixed Events

- Random background estimated by mixed events technique
- Pair combinations of photons from different events
- By construction, photons cannot originate from same p0
- Random minv distribution
- In p+p: Fit of random background good enough

Mixed Events II

- minv distribution for real and mixed events
- Mixed event distribution has to be normalized to real event distribution
- In principle one knows the normalization
- In practice much too complicated to know
- # pairs real: n(n-1)/2
- # pairs mixed: n*m
- n, m vary event by event
- Would need to keep track of all n, m
- Correlated pairs in peak in real events become uncorrelated pairs in mixed events
- Would require iterative procedure to calculate normalization (and peak content)
- Other correlations in real events whose size is not known (η, other resonances, back-to-back correlations, non-vertex p0’s, HBT)

Normalization simply from real/mix ratio

outside peak region, but close to it

!

Constant for syst. error

p0

Invariant Mass [GeV/c2]

Invariant Mass [GeV/c2]

Invariant Mass [GeV/c2]

Yield Extraction

Example : p0 d+Au

Real/Mix

Real

normalized Mix

Real -

normalized Mix

pT =1-1.5 GeV/c

Invariant Mass [GeV/c2]

Invariant Mass [GeV/c2]

Invariant Mass [GeV/c2]

Yield Extraction

Example : h p+p

Real/Mix

Real

normalized Mix

Real -

normalized Mix

pT =4-4.5 GeV/c

Invariant Mass [GeV/c2]

Invariant Mass [GeV/c2]

Invariant Mass [GeV/c2]

Yield Extraction

Example : h p+p

Real/Mix

Real

normalized Mix

S/B: 0.21 – 2.0

Real -

normalized Mix

pT =4-4.5 GeV/c

h min bias Au+Au

Real/Mix

Real

normalized Mix

S/B: 0.002 – 1.5

Real -

normalized Mix

pT =3-4 GeV/c

Invariant Mass [GeV/c2]

- It’s not at the mass !
- Natural with of p0 peak is 7.7 eV -> negligible
- Measured width comes from limited energy resolution of detector
- Random up and down fluctuations of energy along with steeply falling spectrum increase the average observed energy in a given bin
- This shifts the p0 peak up

- A fast Monte Carlo
- Generate photons and p0’s randomly according to assumed pT and y distribution
- Smear energy and position of photons
- Shower overlap:
- Decide randomly for each photon whether overlap takes place
- model PID cut losses by energy dependent photon survival functions
- Tuning
- tune smearing and overlap probability by comparing p0 peak position and width from Fast MC to real data
- estimate PID cut losses by comparing raw p0spectra for different PID’s

- Fast MC: fast, good description if occupancy low
- limitations in central Au+Au

!

- A full simulation
- detector response of single photons or p0’s simulated with GEANT
- single particle response embedded into measured events before reconstruction
- Assumption: no significant change of event properties
- reconstruct event
- compare pT of embedded particle after reconstruction to input pT
- Tuning
- Adjust energy and position resolution to match p0 peak position and width in real data

Full simulation difficult to maintain

Learn from full simulation,

use fast implementation as soon as possible

!

2 GeV

3 GeV

10 GeV

8.5 GeV

15 GeV

p0 p+p

total

7 %

12 %

17 %

E- scale

5 %

9.4 %

11 %

h Au+Au

total

14 %

38 %

yield extr.

10 %

37 %

p0 Au+Au

total

19 %

19 %

efficiency

11 %

11 %

E- scale

11 %

11 %

The Big C…

It’s an established procedure…

The same procedure as every year?

(Un-)Fortunately not!

C

C

entrality - Bias

C

hoice of events - Trigger

C

C

alibration

C

heck of data quality

C

ross section

Choice of events

- PHENIX uses various high pT triggers
- In/without coincidence with main trigger (BBC)
- Advantage for fast analyses:
- Filtering of data
- Splitting of data in smaller nDSTs, pDSTS…
- Easier to handle
- No overhead of “useless” events
- Important: Synchronise smaller DST units

Create small subsets of the DST with sharp event

selection for distinct analyses

!

Triggered events: Mixing

- Always one high-pT photon in each event
- Event mixing: high-pt photon in each of the two events
- Pair of two high-energy photons—a very unlikely case for real events
- minv distribution is biased, does not match the random background in real events
- Solution: Triggered events are mixed with MB events
- In the age of filtered triggered events
- Minimum bias events are often not readily available
- Pseudo minimum bias events
- In one of the triggered events the photon that triggered the ERT is not used for mixing

Calibration

- Often the calibration in the DST is not good enough
- p0peak position can be used to get both
- relative (tower-by-tower)
- absolute calibration
- Relative calibration
- Fill minv disribution for each tower
- Balance tower gain factors so that all peaks are at the same position

Make sure in the DST (+derivatives) you have all

the information to correct the calibration

!

Calibration II

- Predict expected peak position (not at 135 MeV!) with simulation
- Tune energy resolution in simulation to match peak width vs pT in data
- Match peak position vs. E in data to prediction from tuned simulation
- To get p0 peak position vs E:
- replace p0 pT by average photon energy and apply tight asymmetry cut

Calibration III

- Information will change
- Changes have to be communicated
- It’s impossible to wait for the “final” correction
- Hide it from the user
- Centralize it for the experts
- Make it impossible to get the wrong data

Make sure to organize afterburner on the

collaboration level

!

!

Central Cu+Cu also suppressed

Consistent with energy-loss calculation dNg/dy = 370

- Geometrical model with “corona” effect
- More jets from surface
- Correlated with ellipticity

Au+Au

30-40%

Npart= 114

Cu+Cu

0-10%

Npart= 98.2

p0s in d+Au

<kT>= 0.52 GeV2

PHENIX preliminary

Accardi, Gyulassy. Partonic Glauber-Eikonal approach: sequential multiple partonic collisions.

Phys. Lett. B 586 (2004) 244.

Comparison to theory

Vitev, Trento 2005

Energy loss in cold nuclear matter to explain forward rapidities

Power corrections( high twist shadowing)

PID: Shower Shape II

Photons

- Dispersion cut
- rejects ~ 50% of hadronic showers
- Keep 98% of photons (PbGl)
- PbSc
- shower shape is compared to typical shape of electromagnetic shower
- Similarity to electromagnetic shower calculated

Pions

Dcorr

PID: TOF

- Photons massless
- travel at speed of light
- Hadrons have mass
- slower than photons
- TOF cut
- rejects hadrons
- keeps photons

More Pitfalls

- Make sure you also exclude η peak
- Make sure you apply cut on minimum distance between two photons
- Two photons cannot come closer than spatial resolution of detector allows in real events
- In mixed events any distance is possible
- If you don’t apply minimum distance cut in mixed events the minv distribution won’t match the one in real events

Corrections

- Geometrical Acceptance
- Not all p0’s hit EMCal
- Limited η and φ coverage
- Towers not used in analysis
- Even if one of the decay photon hits, the other might still miss
- Opening angle of photon pair is pT dependent
- Acceptance is pT dependent

Calculate Acceptance

- MC simulation: generate p0’s and photons
- |h| < 0.45 (account for vertex variation)
- Gaussian Rapidity distribution
- calculate kinematics of p0 decay photons
- take inactive detector areas into account

Efficiency

- Definition:

- Corrects for detector effects like
- limited resolution
- shower overlaps
- photon losses due to PID cuts
- Two approaches: Fast MC, full simulation

p+p, Fast MC

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